Petrogenesis of the Llano Granites, Central Texas

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Petrogenesis of the Llano Granites, Central Texas
Elizabeth Roller December 3, 2002
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Topics to be Covered

• Geologic setting
• Why do we care?
• Creating a map
• Creating a dataset
• Problems encountered
• Further work

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Tectonic Setting

• Grenville Orogeny - 1.2.-1.1 Ga
• Nearly every continent has an orogenic belt
  dating to the Grenville time period
• Llano is the southernmost extent of the Grenville
  in the continental US.

From Patchett and Ruiz, 1989
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Tectonic Setting, cont.
From Mosher, 1998
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Late to Post Tectonic Granites

• Cover over 42 of the Llano Uplift
• Data implies intrusion during the waning stages
  of orogenesis.
• Origin is poorly known

Granites in the Llano Uplift
From Reed, 1999
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Why do we care?

• These types of granites (A-type) are found
  associated with many other orogenic belts.
• A debate exists on the genesis of A-type granites
• Radioactive isotope analyses (Sm-Nd, Rb-Sr,
  Pb-Pb) can constrain formation mechanisms
• Very little isotopic data exists for this area

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Creating a Map

• No digital geologic map exists for the Llano
  uplift area
• The following were available to me paper
  geologic map geospatially oriented raster of ½
  the above map CAD drawing Texas zip code map
  (GIS File)
• Used the Transformations tab under Properties
  to crudely orient CAD drawing

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Unoriented CAD drawing with very nice layers and
fault lines and full coverage
Oriented raster file with no layers and only half
coverage
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Two point transformation utilizing two points
where the coordinates were known on both the CAD
drawing and the raster file.
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Creating a Dataset

• I wanted to put the following in my dataset for
  each sample collected

• Existing geochemical data
• Existing isotopic data, including U-Pb dates
• Geospatial Data

BACK to GIS Project
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This was accomplished by exporting an excel file
to a .dbf file, and then using the display X-Y
data function.
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Distribution of Rare Earth Elements within the
High Y, Zr granites
Graph of the distribution of the Rare Earth
Elements within one of the types of granites
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Problems Encountered

• The two-point transformation did not work as well
  as I would have liked and the raster plot only
  has half the data of the CAD drawing
• A few of my X-Y data plotted off the map, which
  was easily fixed by going back into the data and
  manually fixing the points.

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Further Work

• Obviously, fix the poor fit of the CAD drawing to
  the raster plot needs to be fixed.
• Geo-referencing more points?
• More data within the sample points